Building maintenance workers were circulating through the office space ensuring power had been restored,which it had.
第一这是一个复杂句
S：Building maintenance workers，其中building maintenance是名词作定语修饰workers
P：were circulating,又助动词的be和动词circulate构成动词词组谓语，同时加入了过去进行时时态。
Ad：through the office place是介词短语作地址状语。
Ad：ensuring power had been restored :我猜测应该是目前分词+宾语构成分词短语作目的状语。而宾语成分却是一个that引导的宾语从句，that被省略了。power had been restored是一个从句，that在从句中不做成分。power 是从句的主语，had been restored是两个助动词+动词restore构成谓语动词组，同时加入时态和被动。
which引导非限制性定语从句,这里应该是修饰，which等于been restored，那他在句子中是作什么成分呢？
直观剖析：
Building maintenance workers （主语）were circulating （谓语动词）through the office space （介词短语作状语）ensuring power had been restored（带有目的的随着状语）, which it had.（定语从句）
【难题解说】本句难题不在于目前分词短语 ensuring power had been restored 的语法用途，而在于定语从句：
it 指什么？ —— 指 power
which 指什么？——指 been restored，不表示其他。
which it had = it （电力）had been restored （已被恢复）
【注】关系代词 which，可以指代主句中的部分成分，甚至是谓语动词的一部分： been restored。
which 的“特殊指代”解说：
关系代词 which，除去指代主句中的某个名词或者整个主句的内容外，还可以代替主句中的名词词组、形容词或形容词短语，甚至指代部分谓语动词。由于 which 可以表达比 that 和 who愈加丰富的内容，这是其他关系代词取代不了的。
1. She is the perfect accountant which her predecessor was not.
她是一个完美的会计师，而她的前女友不是。
which 不单纯指 accountant，而是整个词组 the perfect accountant。
2. The man was unable to earn enough money, which allowed him to support his family.
那人没办法赚到足够的钱，足够的钱才能让他得以养活家人。
（which 指代enough money，不是 money，更不是整个主句：既然赚不到钱，后面的句子就不通了。）
3. He was notsick, which some of the other passengers were.
他没生病，但有的其他旅客却病了。
（which 指代形容词 sick）
4. She is always careless, which we should not be.
她一直粗心大意，大家可不应该如此。
（which 指代形容词 careless）
5. The two policemen were completely trusted, which in fact they were.
那两个警察完全遭到信赖，事实上，也真是这样。
（which 指代形容词短语 completely trusted）
6. She was very patient towards the children, which her husband seldom was.
她对孩子们非常耐心，她老公却极少如此。
（which 指代形容词短语 patient towards the children）
